Determining the size of the garage you actually need will depend on several factors. Here is a handy checklist of things you should think about that will help determine what size will best suit your needs. How many vehicles do you have? Will it be one story or two? Do you want an attached garage or a free standing one? Will you be using the garage for anything beside storage? Do you want to include a work bench? How many garage doors will you need?
Take accurate measurements of 
your vehicle. The minimum suggested building length is 14 feet for a car, 18 feet for a pickup truck and 20 to 22 feet for a full-size truck or SUV. The most common garage sizes are 12×24, 14×28, 20×20, and 24×24 feet. That means that a one-car garage should be at least 20×20. Plan on adding another four feet if you need some storage. If you have a handyman in the family who wants space for a workbench, you’ll need an extra 10 feet of space. A 24×28-foot building will be large enough to hold a regular-sized vehicle and a work area.
Once you settle on the size of your garage, make sure the doors will be large enough and convenient to use. Many single garage doors are eight feet wide. You could be in for a tight squeeze if your car is bigger than six and a half feet wide. That only leaves you about nine inches of leeway on the sides and isn’t very mirror friendly if you know what I mean. You might want to buy nine-foot doors for the extra room, which won’t lop off your mirrors if you miscalculate when pulling in. And if you are planning on installing a double-width door, consider using one that is 18-feet wide as opposed to the standard 16-foot ones. Keep those mirrors on your car. They’re pretty helpful!